The present invention relates in general to a new and improved article of manufacture especially for use as a protective shield and the method of making the same and in particular to a window transparent to electromagnetic radiation comprising an aluminum nitride-boron nitride composite.
Modern high-speed vehicles such as high-speed aircraft, may be required to pass through a hostile environment. Under these conditions, it may become necessary to provide protection for certain communication and detection apparatus carried by such a vehicle from sources of heat shock and pressure shock. In a high-speed vehicle of the kind under discussion, such shock is typically due to frictional contact with the ambient environment. Protection from heat and pressure shock must be provided without detracting from the ability of the protected apparatus to communicate with the outside of the vehicle by means of electromagnetic radiation within the frequency spectrum of interest. A common way of providing the desired protection is to place the apparatus behind a suitable protective shield or window, which is transparent to the desired frequency spectrum of electromagnetic radiation, sometimes referred to as an electromagnetic window.
Presently available compositions for providing such an electromagnetic window often lack the requisite mechanical strength, toughness and thermal shock resistance. Fused silica is used extensively for electromagnetic windows because it has exceptional radar transparency, high thermal shock resistance and desirable dielectric properties. However, because of its low mechanical strength and toughness, its low spall resistance and its undesirable recession rate, the use of fused silica as a shield for apparatus of the kind discussed above, e.g. for use as an electromagnetic window, has proved to be unsatisfactory in a number of respects.
A material for use as an electromagnetic window, which fulfills more completely the demands placed on it, must improve on the performance of existing materials in their function of shielding and protecting the communication and detection apparatus inside the vehicle, while allowing electromagnetic radiation in the desired frequency spectrum to pass. Ideally, a structure which consists of such an improved material will possess the following attributes: radar transparency, congruent vaporization kinetics, tailorable dielectric properties, thermal shock resistance, mechanical strength and toughness, spall resistance, high refractoriness, and a tailorable recession rate, i.e. erosion rate and ablation rate. Compositions having these properties though of course suitable for other uses, can provide protective shields and covers especially suitable for use as electromagnetic windows in a hostile environment.